Blade comprising a support, provided with a portion with a depression

ABSTRACT

The support ( 6 ) of this blade comprises one side ( 7 ) and the other ( 18 ) provided with a depression ( 10 ). By concentrating this depression on a single side, a support ( 6 ) that is more rigid and resistant with an equal lightening is obtained, with the stress concentrations depending on the depth of the depression but being absent from the flat side ( 7 ). The vibrations and bending deformations are also reduced. Application to blades of a turbomachine, in particular compressors.

This invention relates to a blade comprising a support provided with aportion with a depression on a single side.

This blade, which can be a turbomachine compressor blade, comprises anoutside vane subjected to aerodynamic forces, and a foot retained in arotating disc; the foot comprises a platform flush with the surface ofthe disc and from which the vane rises, a bulb retaining the bladeagainst the centrifugal forces and retained in an enlargement of agroove of the disc, and a support joining the bulb to the platform, andhoused in a main portion of the groove.

Portions with a depression have sometimes been dug into the main orlateral faces of the support so as to lighten the blade. Disadvantages,such as local increases in the static stresses and an alteration in thefrequency of the bending modes, have however been observed with respectto blades of which the supports are devoid of a depression.

In French patent application 12 60527, it was undertaken to reduce thesedisadvantages and to bring the behaviour of the blade provided withportions with a depression closer to that of a blade with perfectly flatsupports, by choosing dissymmetric portions with a depression, inparticular with regards to the depth. This application clearly specifiesthat the portions with a depression are established on the two mainsides of the support.

Document U.S. Pat. No. 5,435,694 A describes a blade wherein the supportis joined to the platform or to the bulb by fillets, which are mouldingswith a slightly concave section, in order to reduce the stress that theblade in service is subjected to. Portions slightly with a depressionare added in order to reinforce this effect, by decreasing theconcentrations of stress around the fillets. These portions with adepression are defined by two successive radii of curvature and aresuperficial. It seems that they are provided on the two sides of thesupport. It is not sought to lighten the blade by means of theseportions with a depression or by fillets.

GB 2 162 588 A describes a blade support provided with a recess thatopens onto one of the end faces of the support in order to lighten it.This design is suitable for relatively thick supports. The structure ofthe support remains symmetrical in the angular direction on either sideof a median plane extending in the length of the support.

The invention relates to a lightened blade, designed to minimise thedegradations in aeromechanical performance, in particular concerning theconcentrations of stress and the frequency of the bending modes.

According to the invention, one of the main faces of the support issubstantially plane or curvilinear—in any case with a regular surfacewithout relief—while the other of the main faces is provided with aportion with a depression, extending over a depth of at least 25%, andadvantageously by at least 35%, of a thickness of the support, betweenthe main faces. It is specified that the main faces of the support—whichextends roughly in the axial direction of the disc and of theturbomachine to which the blade belongs—are the lateral faces with thelargest surface area, roughly perpendicular to the angular direction ofthe disc and of the machine, giving onto the intrados and extrados sidesof the blade, and extending in the direction of insertion of the foot ofthe blade into the cutout of the disc.

The degradation in the performance obtained on the blade is then clearlyless substantial than with portions with a depression distributedsymmetrically on the two sides of the support, or even than with theportions with a depression opposite and moderately dissymmetricconsidered in application FR 12 60527. And GB 2 162 588 does not suggestthat a section of dissymmetric support in the angular direction wouldimprove the rigidity of the support in this direction wherein thebending of the blade is exerted.

Indeed, flatness maintained on one of the main faces increases therigidity of the blade with regards to the deformations in bending andprevents the introduction of concentrations of stress on one side: itcan therefore be considered to arrange the depression on the side wherethe concentrations of stress are the least feared, i.e. the side that isthe least loaded in the critical situations to be considered, such asthe loss of a blade, in order to obtain a globally satisfactorydistribution over the blade by not weakening the other side.

According to other optional improvements, the portion with a depressionhas a bottom that is substantially flat or curvilinear (parallel to themain face, and therefore at a uniform depth), surrounded by a curvededge, connecting to the bottom without forming an angle, in order toobtain a good lightening without favouring concentrations of stress atthe edge of the portion; where it is limited by rectilinear sides joinedby curvilinear connectors.

The invention shall now be described in liaison with the followingfigures:

FIG. 1 is a general view of the blade in the angular direction; and

FIG. 2 is a view of the foot of the blade in the axial direction.

FIG. 1 shows a blade in accordance with the invention, provided with avane 1 to the exterior and with a foot 2 engaged in the rotating disc 3,with the foot 2 comprising a platform 4 joined to the vane 1, a bulb 5opposite the platform 4, and a support 6, joining the two latter items.As the platform 4 is conical, the support 6 is trapezoidal.

The support 6 comprises two main faces 7 and 8 opposite, shown in FIG.2, giving onto the intrados and extrados sides of the blade andextending roughly in the direction of the axis of rotation of the disc3. The main face 7 is flat (smooth, linearly or curvilinearly flat), butthe main face 8 is provided with a portion with a depression 10, takinga flat bottom 12 and an edge 14 connecting the bottom 12 to the mainface 8. The bordure 14 is connected to the bottom 12 without forming anysharp edge, but with a progressive rounding. The portion with adepression 10 is of trapezoidal shape, as shown in FIG. 1, with a firstlongitudinal side 15, close to the bulb 5 and parallel to its junctionwith the support 6, a second longitudinal side 16 close to the platform4 and parallel to the connection between it and the support 6, and twoother sides 17 and 18, connecting the latter together and curvilinear atleast at the ends.

The portion with a depression 10 can be present according to case on theintrados or the extrados side of the blade.

By making use of a depression on a single side of the support 6, thegain in mass is provided while still minimising the effect on theaerodynamic and mechanical behaviour of the blade: the static stress,the position of the first bending mode and the response of the firstbending mode under substantial stress, are improved with respect to ablade comprising, for the same lightening, portions with a depression onthe two sides.

In relation to such a blade comprising two portions with a depression,it has been observed, in a design in accordance with the invention,wherein the thickness of the support 6 (distance between the two mainfaces 7 and 8) was 15.4 mm and the portion with a depression had a depthof 5.4 mm, a static stress at the output of the blade, on its contactsurfaces with the disc 3, reduced by 5% in the most loaded zone withrespect to a blade having depressions on the two sides and a similarlightening; and for a first bending mode, the deformations measured inthe support 6 were reduced by 30% following substantial stress such asfloating (instability due to the interaction of the fluid with thestructure of the vanes 1 which has for effect to amplify the vibrationof the blades), ingestions of foreign bodies or the loss of a vane 1.

It is therefore possible to hope for a better resistance, or a longerresistance in the case of fatigue, of the foot 2, and better resistanceof the vanes 1 with regards to vibrations. It is to be added that thefrequency of the bending mode is not modified by the concentration ofthe lightening of a single side, in such a way that the invention doesnot require having to again optimise the blade with regards to thesefrequencies, once the optimisation has been carried out for blades withsupports recessed on the two sides.

The same considerations apply to blade supports of which the main faceswould be curvilinear instead of being flat: the portion with adepression would again be located on only one of its main faces andwould be dimensioned in the same way.

1. Blade, in particular of a compressor, comprising a vane (1) and afoot (2), with the foot comprising successively, by moving away from thevane, a platform (4) joined to a support (6) and a bulb (5), with thesupport (6) comprising two main faces (7, 8) respectively directed in anintrados and an extrados side of the blade and parallel, characterisedin that one of the main faces (7) is substantially plane or curvilinear,while the other of the main faces is provided with a portion with adepression (20) extending over a depth of at least 25% of a thickness ofthe support between the main faces.
 2. Blade according to claim 1,characterised in that the portion with a depression extends over atleast 35% of the thickness of the support.
 3. Blade according to claim1, characterised in that the portion with a depression has a bottom (12)that is substantially flat or curvilinear surrounded by a curved edge(14) that connects to the bottom without forming an angle.
 4. Bladeaccording to claim 2, characterised in that the portion with adepression has a bottom (12) that is substantially flat or curvilinearsurrounded by a curved edge (14) that connects to the bottom withoutforming an angle.
 5. Blade according to claim 1, characterised in thatthe portion with a depression is limited by rectilinear sides joined bycurvilinear connectors.